This invention relates to a permeable membrane having a porous substrate and a functional surface layer on the surface of and/or inside the porous substrate. This invention particularly relates to a permeable membrane suitably used for allowing the selective permeation or separation of a gas in the presence of highly polar compounds such as a vapor of water, ethylene glycol, xcex3-butyrolactone or ammonia for example.
This invention also relates to particular applications of the permeable membrane. The permeable membrane of this invention can be used, for example, in electrolytic capacitors and fuel cells. It allows the selective permeation of any of various gases and liquids even in the absence of a highly polar compound vapor. If the permeable membrane of this invention is used in a sealed electrolytic capacitor otherwise likely to be damaged by the pressure of the hydrogen gas generated inside the sealed container, such damage can be avoided and the capacitor life can be sufficiently extended, and the safety of the electrolytic capacitor during use can also be secured.
A fuel cell uses hydrogen as a fuel, and the hydrogen is obtained by reforming methanol or methane. The reformed gas contains water, carbon monoxide and carbon dioxide in addition to hydrogen. A membrane that allows the permeation of hydrogen but not carbon monoxide avoids poisoning the electrode catalyst by carbon monoxide. This is important. Unless the membrane causes selective permeation of hydrogen rather than water, water would be adsorbed in the pores of the membrane, reducing the permeating quantity of the hydrogen. Therefore, a membrane that allows selective permeation of hydrogen in preference to water is very important for gas separation in the presence of water vapor. Since water is smaller than hydrogen in its minimum molecular size (kinetic diameter), it is very difficult to achieve selective permeation of hydrogen in preference to water vapor.
A palladium membrane is known as a membrane allowing selective permeation of hydrogen. However, palladium is very expensive, and it is difficult to use it industrially.
In the field of electrolytic capacitors, Japanese patent Applications JP, 62-112314, A and JP, 62-272515, A propose, for example, methods of discharging hydrogen gas generated in an electrolytic capacitor outside using a hollow-yarn permeable membrane made of a polyimide, polytetrafluoroethylene or polypropylene, etc., but those methods do not sufficiently discharge hydrogen gas or prevent the increase of internal pressure. Furthermore, an electrolytic capacitor has a significant disadvantage that since the electrolyte permeates as a vapor from the electrolytic capacitor, the composition of the internal electrolyte changes, and thereby changes the characteristics of the electrolytic capacitor.
Furthermore, in recent years, membranes using a zeolite and/or a zeolite analog have been positively studied, but it is difficult to obtain a zeolite membrane having specific permeation characteristics with high reproducibility. Moreover, there has heretofore been no proposal of (1) a membrane allowing the selective permeation of hydrogen instead of water vapor from a mixture containing water vapor and hydrogen, (2) a membrane causing good permeation of hydrogen but resistant to allow the permeation of ethylene glycol, and (3) a zeolite and/or a zeolite analog membrane allowing good permeation of hydrogen but resistant to allow the permeation of ammonia.
An object of this invention is to overcome the disadvantages of the above prior art by providing a permeable membrane not substantially containing any expensive metal having affinity with hydrogen, excellent in heat resistance and chemicals resistance, and allowing good permeation of hydrogen, and (1) resistant to permeation of water vapor instead of hydrogen, (2) resistant to permeation of ethylene glycol vapor, or (3) resistant to permeation of ammonia gas.
Another object of this invention relates to an application of the permeable membrane and the related art by providing an electrolytic capacitor having said permeable membrane for effectively utilizing the permeability of the permeable membrane.
A further other object of this invention is to provide a method for producing a zeolite membrane preferably used as the functional layer of said permeable membrane, and an MFI zeolite membrane as an embodiment of the zeolite membrane.
A further other object of this invention is to provide a method for separating at least one kind of molecules by using said permeable membrane or MFI zeolite membrane for allowing the permeation of at least one kind of molecules from a gas or liquid mixture consisting of at least two kinds of molecules.
The inventors studied intensively to achieve the above objects, and as a result, arrived at this invention having the following constitution.
The subject matter of this invention is a permeable membrane, (a) comprising a porous substrate and a functional layer provided on the surface of and/or inside the porous substrate, (b) having a hydrogen permeation rate of 1xc3x9710xe2x88x9210 m2xc2x7secxc2x7Pa or more, and (c) satisfying at least one of the following conditions:
(1) Hydrogen is selectively allowed to permeate rather than water vapor.
(2) When a test tube containing ethylene glycol and sealed with said permeable membrane is placed in an 85xc2x0 C. oven, the ethylene glycol decrease rate measured is 1xc3x9710xe2x88x922 g/hour or less per 1 cm2 of the permeable membrane.
(3) When a test tube containing 5% ammonium adipate ethylene glycol solution and sealed with said permeable membrane is placed in an 85xc2x0 C. oven for 2 days, the acid quantity of the solution is 5xc3x9710xe2x88x923 equivalent or less per 1 cm2 of the permeable membrane.
It is preferable that the functional layer of the permeable membrane of this invention contains at least one of the following ingredients (1) through (5):
(1) A zeolite and/or zeolite analog
(2) Fine inorganic oxide grains
(3) A silicone rubber, silicone resin or silicone oil
(4) An organic macromolecular compound
(5) Carbon
The permeable membrane of this invention includes the following as preferable modes:
(1) The functional layer as the outermost surface layer is covered with a silicone compound and is made of a zeolite and/or zeolite analog.
(2) The zeolite and/or zeolite analog is high silica and/or pure silica zeolite.
(3) The permeable membrane is 2xc3x9710xe2x88x927 mol/m2xc2x7sxc2x7Pa or less in the difference between the hydrogen permeation rate at room temperature after calcining the permeable membrane and the hydrogen permeation rate at room temperature immediately after keeping 85xc2x0 C. ethylene glycol saturated vapor in contact with the permeable membrane for 24 hours.
Further subject matter of this invention is an electrolytic capacitor using said permeable membrane. The capacitor includes preferable modes that the permeable membrane is mounted at a sealing plug with an elastic body intervening between the permeable membrane and the sealing plug, and that the permeable membrane has an inorganic porous substrate and a functional layer.
Further subject matter of this invention is a method for producing a membrane containing a zeolite or zeolite analog, comprising the step of treating a membranous substance containing a seed crystal of a zeolite or zeolite analog and silica, with water vapor. In the production method of this invention, it is preferable to bring a seed crystal of a zeolite or zeolite analog into contact with a porous substrate beforehand, and to coat the porous substrate with a solution, slurry or colloid containing silica, for forming a membranous substance, and then to treat with water vapor. In this case, it is preferable that the water vapor treatment temperature is 80 to 200xc2x0 C.
Further subject matter of this invention is an MFI zeolite membrane satisfying the following two conditions:
(1) a/b=0.3 to 1.5
(2) b/c greater than 4.4
where a is the maximum peak intensity within a 2xcex8 range of 7.3 to 8.3 degrees, b is the maximum peak intensity within a 2xcex8 range of 8.5 to 9.1 degrees, and c is the maximum peak intensity within a 2xcex8 range of 13.0 to 14.2 degrees, respectively in the diffraction pattern obtained by X-ray diffractometry using CuKxcex1 as the X-ray source with the incident angle fixed at 3 degrees at a scanning speed of 2xcex8 4 degrees/min in a parallel optical system.
Further subject matter of this invention is a method for separating at least one kind of molecules by bringing a gas or liquid mixture composed of at least two kinds of molecules into contact with the permeable membrane or MFI zeolite membrane, for allowing the preferential permeation of at least one kind of these molecules.